Tezlink/node: mock expected issuance

What

We mock one of the adaptive issuance service.

Why

It's called by Tzkt but it's not really relevant to tezlink so we try a mock.

How

We can't directly import the service as it's not exposed in Rio, just in Alpha, so we copy the definition.

We return dummy values, and assume we are in a never-ending cycle 0 for now. The goal is to move forward until we can properly test tzkt, then to adjust the mock if necessary.

Manually testing the MR

There's a test, because it's not hard to write, but it's not super informative except for encodings maybe ?

 dune exec etherlink/tezt/tests/main.exe -- tezlink /threshold_encryption /dal mock

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR